BCR and infection: Tonsil cells from a separate group of pediatric subjects who were either post-infection or had hybrid immunity to SARS-CoV-2 (both infected and vaccinated, Supplemental Table 2) were stimulated with soluble anti-human IgA, IgG, and IgM for 2 minutes and phosphorylation of the tyrosine kinase Syk and phospholipase Cg2 (PLC-g2), two downstream signaling proteins rapidly phosphorylated upon BCR stimulation, were measured71.